Which one of the following ligands forms a chelate?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Oxalate
A chelate is formed when a ligand binds the same metal ion through two or more donor atoms at the same time, so that a ring containing the metal is formed. Only a didentate or polydentate ligand can do this.
The oxalate ion, C2O42–, has two O– donor atoms that bind the metal together and form a five-membered ring, for example in [Fe(C2O4)3]3–. Chelate complexes are more stable than similar complexes of unidentate ligands.
Why the other options are incorrect
Option (1) Cyanide — Incorrect. CN– is unidentate; it can bind through C or through N, but only one atom at a time, so it cannot close a ring.
Option (2) Water — Incorrect. \( \ce{H2O} \) binds through its single oxygen atom; it is a unidentate ligand.
Option (4) Chloride — Incorrect. Cl– has one donor atom and is unidentate.
NCERT (2026-27 edition):
Definitions of Some Important Terms Pertaining to Coordination Compounds – …
